[Note] LUA Hello World
===
###### tags: `note`, `lua`, `helloworld`
[toc]

# Introduction
## Features
> Lua (/ˈluːə/ LOO-ə; from Portuguese: lua [ˈlu.(w)ɐ] meaning moon) is a
> - cross-platform
> - dynamic language
> - embedable
> - extensible
> - high-level
> - lightweight
> - multi-paradigm programming language
> - open source
> - portable
> - dynamic language
> - weak typing
> - extensible
> - embedable
> - portable

## Interpreter
> Lua's interpreter of compiled bytecode (about 100 KB) is written in ANSI C,and Lua has a relatively simple C API to embed it into applications.

## Application
Lua's application includes
- embedded system such as Android and iOS
- PSP and PS3
- Node NCU
- Wireshark
- Snort
- Games; LucasArts and World of Warcraft
- Game engine; Corona SDK and Moai

# Installation

[TomasPC]# curl -R -O http://www.lua.org/ftp/lua-5.3.4.tar.gz
[TomasPC]# tar zxf lua-5.3.4.tar.gz
[TomasPC]# cd lua-5.3.4
[TomasPC]# make linux test
[TomasPC]# lua -v
Lua 5.1.4 Copyright (C) 1994-2008 Lua.org, PUC-Rio

# Syntax

## Data Type
There are total 8 kinds of data type including **nil, boolean, number, string, userdata, function, thread and table** as shown below table.

| DataType | Description |
| -------- | ----------------------------------------------- |
| nil | null and is false |
| boolean | ture or false |
| number | double in C |
| string | "a string" or 'a string' |
| userdata | any C structure stored in memory |
| function | any C or Lua function |
| thread | a single thread |
| table | an array and the index can be integer or string |

### Array

arr= {"h", "e", "l", "l", "o"}
for index= 1, #arr do
    print(arr[index])
end

for i,v in ipairs(arr) do
    print(i,v)
end

### Function
It is allowed to return many values of a function.

-- Example 1
function calc(a,b,c)
return a b c
end

result= calc(1,2,3)
print(result)

-- Example 2
function getCalc(a,b,c)
return a,b,c
end

r1,r2,r3= getCalc(1,2,3)
print(r1,r2,r3)

### Table
Table is a data structure in Lua like a map in Java or JSON object in Javascript.
The size of Lua table is not flexible.

person ={}
person.name ="jack"
person.age =20

print(person[1])
print(person.name)
print(person["age"])

## Comment

### Single line

-- A single comment

### Block

--[[Comments can be spread
across several lines ]]
print "Hello World!"

### Nested
Nesting of comment blocks will cause a syntax error.

--[[
  This will not work. Nesting of comment blocks will cause a syntax error.
  --[[
    By nesting existing code containing a comment, a syntax error will occur. This is because the nested symbols will confuse the interpreter, so a modification is required to remove the symbols
  ]]
  print "Hello"
]]

## Assignment
The default type of variable is nil and global.

a = 10
str = "hello world"
a, b = 10, 15
c = a + b -- plus
d = a - b -- substract
e = a * b -- multiply
f = a / b -- divide
g = a % b -- mod
h = a^2   -- square
i = -a    -- negative

## Operation

a , b = 10 , 15
print(a > b)  -- greater than
print(a < b)  -- less than
print(a == b) -- equal to
print(a ~= b) -- not equal to

print(true and false) -- and
print(true or false)  -- or
print(not true)       -- not

## Flow control
### if condition

-- if expression
if(true)
then
print("hello")
end

-- if .. else expression
a, b= 10, 15
if(a> b)
then
print(a)
else
print(b)
end

-- nested if
c= 20
if(c >10)
then
    if(c <30)
    then
    print(c)
    end
end

### loop condition

| loop type |
| ---------------- |
| while |
| for |
| repeat ... until |
| nested loop |

-- while loop
a=10
while( a> 0)
do
    print(a)
    a =a -1
end

-- for loop
for a=1, 10, 1 do
    print(a)
end

-- repeat ... until loop
a=10
repeat
    print(a)
    a= a- 1
until(a<1)

### Module and Package
Establish a file named module.lua.

module={}
module.index=1
function module.sum(a,b)
return a b
end

Import from the other file.

-- "module" here is a file name.
require "module"

-- module here is the name of defined in the imported module
print(module.index)
print(module.sum(1,2))

# Reference
http://www.lua.org/
http://www2.kimicat.com/lua%E6%95%99%E5%AD%B8
http://yhhuang1966.blogspot.com/2015/07/lua_14.html
https://en.wikipedia.org/wiki/Lua_(programming_language)
https://codertw.com/%E5%89%8D%E7%AB%AF%E9%96%8B%E7%99%BC/390702/
https://en.wikibooks.org/wiki/Lua_Programming